A Traveling Wave Tube (TWT) or a klystron is an electron tube used for amplifying or oscillating a high-frequency signal through interaction between a beam of electrons emitted from an electron gun or the like and a high-frequency circuit.
Referring to FIG. 1, a TWT 1 includes electron gun 10 for emitting a beam of electrons 50, helix electrode 20 functioning as a high-frequency circuit that allows a beam of electrons 50 emitted from electron gun 10 to interact with a high-frequency signal (i.e. a microwave signal), collector electrode 30 for collecting beam of electrons 50 emitted from helix electrode 20, and anode electrode 40 for drawing out electrons from electron gun 10 as well as guiding the beam of electrons 50 emitted from electron gun 10 into helix electrode 20. Electron gun 10 has cathode electrode 11 for emitting thermal electrons and heater 12 for supplying thermal energy for causing emission of the thermal electrons.
The beam of electrons 50 emitted from electron gun 10 is accelerated by the electric potential difference between cathode electrode 11 and helix electrode 20 before entering helix electrode 20, and then travels inside helix electrode 20 while interacting with the high-frequency signal inputted through one end of helix electrode 20. After the beam of electrons 50 has passed through helix electrode 20, collector electrode 30 captures the beam of electrons 50. Here, the high-frequency signal, amplified through interaction with the beam of electrons 50, is outputted through the other end of helix electrode 20.
Power supply apparatus 60 supplies a helix voltage Ehel, which is a negative direct voltage based on the potential HELIX of helix electrode 20, to cathode electrode 11. In addition, power supply apparatus 60 supplies a collector voltage Ecol, which is a positive direct voltage based on the potential H/K of cathode electrode 11, to collector electrode 30, and supplies a heater voltage Eh, which is a negative direct current based on the potential H/K of cathode electrode 11, to heater 12. In general, helix electrode 20 is connected to a case of TWT 1 and is thereby grounded.
While FIG. 1 illustrates an example construction of TWT 1 having one collector electrode 30, TWT 1 may have a plurality of collector electrodes 30. Although FIG. 1 illustrates a construction in which anode electrode 40 and helix electrode 20 are connected inside power supply apparatus 60, it can be constructed such that anode electrode 40 is supplied with an anode voltage Ea, which is a positive voltage with respect to the potential H/K of cathode electrode 11.
Helix voltage Ehel, collector voltage Ecol, and heater voltage Eh are generated using, for example, a transformer, an inverter and a rectification circuit. The inverter serves to convert a direct voltage supplied from the outside into an alternating voltage and is connected to a first coil of the transformer. The rectification circuit serves to convert an alternating voltage outputted from a second coil of the transformer into a direct voltage.
Since TWT 1 draws out electrons from cathode electrode 11 using the potential difference between anode electrode 40 and cathode electrode 11, it is preferable that the potential difference between anode electrode 40 and cathode electrode 11 be as small as possible in a state where an instable voltage is supplied to respective electrodes at the time when helix voltage Ehel or collector voltage Ecol is raised (inputted).
When there is a potential difference between anode electrode 40 and cathode electrode 11 at the time when helix voltage Ehel or collector voltage Ecol is inputted, a portion of electrons drawn out from cathode electrode 11 flows through helix electrode 20 to a ground potential. This causes an excessive amount of current to flow through helix electrode 20, thereby causing deterioration or damage to TWT 1. In particular, in the construction in which anode electrode 11 and helix electrode 20 shown in FIG. 1 are connected to each other, a potential difference occurs at the same time when helix voltage Ehel is inputted. It is preferable to reduce the potential difference using any means.
As an attempt to avoid such a problem, Japanese Laid-Open Patent Application No. 2005-093229 (hereinafter, referred to as patent document 1) discloses a construction for controlling the supply and cutting-off of an anode voltage using a circuit, which is implemented with a Field Effect Transistor (FET).
FIG. 2 is a block diagram illustrating the construction of a high-frequency circuit system disclosed in patent document 1.
As shown in FIG. 2, the high-frequency circuit system disclosed in patent document 1 includes transistor Q1 and a transistor Q2 provided for on/off control of transistor Q1. In transistor Q1, a source is connected to a cathode electrode of TWT 1 and a drain is connected to a helix electrode via anode electrode A of TWT 1 and via resistor R1. Here, transistor Q1 is an N-channel FET, and transistor Q2 is an N-channel Metal-Oxide Semiconductor FET (MOSFET).
Transistor Q1 has a gate connected to a drain of transistor Q2, and resistor R2 is connected in parallel between the gate and the source of transistor Q1. Transistor Q2 has a source connected to a heater of TWT 1. A gate of transistor Q2 is applied with a voltage, which is obtained by dividing a voltage between the helix electrode and the heater of TWT 1 using resistors R3 and R4.
According to this construction, in a time period when helix voltage Ehel and collector voltage Ecol are being raised (i.e., inputted), transistor Q1 is switched on so that the potential of anode electrode A is substantially identical with the potential of cathode electrode H/K. When helix voltage Ehel and collector voltage Ecol are raised to a predetermined level, transistor Q1 is switched off so that the potential of anode electrode A is substantially identical with ground potential HELIX. Timing to switch transistor Q1 from “on” to “off” is determined by the voltage division ratio between resistors R3 and R4 connected to the gate of transistor Q2.
In the high-frequency circuit system shown in FIG. 2, as shown in FIG. 3, only a faint amount of current IHELIX flows to the helix electrode of TWT 1 when transistor Q1 is switched from on to off. As a result, this can prevent an excessive amount of current from flowing to the helix electrode, as otherwise an excessive amount of current would deteriorate or damage TWT 1. In the meantime it should be understood that the characteristics of the anode voltage and the helix voltage schematically represent changes in the voltages based on the potential H/K of the cathode electrode but are not actual voltage values.
Other attempts to reduce the potential difference between the anode electrode and the cathode electrode when helix voltage Ehel and collector voltage Ecol are inputted are disclosed, for example, in Japanese Laid-Open Utility Model Application No. S57-186966 (hereinafter, referred to as patent document 2), Japanese Laid-Open Utility Model Application No. S61-157251 (hereinafter, referred to as patent document 3) and Japanese Laid-Open Utility Model Application No. H04-076240 (hereinafter, referred to as patent document 4).
As shown in FIG. 4, patent documents 2 through 4 disclose a construction in which resistors R11 and R12 are connected in series between the helix electrode and the cathode electrode of TWT 1 and resistors R11 and R12 supply a voltage obtained by dividing helix voltage Ehel.
In the construction as shown in FIG. 4, when compared with the construction shown in FIG. 1 in which the anode electrode and the helix electrode of TWT 1 are connected to each other, a current flowing through the helix electrode can be reduced when helix voltage Ehel and collector voltage Ecol are inputted. This is because the potential difference between the anode electrode and the cathode electrode is reduced.
As described above, in the construction shown in FIG. 1 in which the anode electrode and the helix electrode of TWT 1 are connected to each other, when helix voltage Ehel and collector voltage Ecol are inputted, an excessive amount of current may flow through the helix electrode thereby causing deterioration or damage to the TWT. The construction shown in FIG. 2 or FIG. 4 is proposed to avoid such a problem.
However, in the construction shown in FIG. 2, since a current flows through the anode electrode when transistor Q1 is open, that is, TWT 1 performs a normal operation, the current also flows through resistor R1 connected between the helix electrode and the drain. Then, as a drawback, the potential of the anode electrode decreases (to a value close to the potential H/K of the cathode electrode), thereby decreasing the maximum gain of TWT 1.
For example, if a current flowing through the anode electrode in the normal operation of TWT 1 is 0.1 mA and if the resistance of resistor R1 is 10 MΩ, the potential of the anode electrode decreases by 1 kV compared to the potential of the helix electrode. If the resistance of resistor R1 is reduced, the potential difference between the anode electrode and the helix electrode can be reduced in the normal operation. However, resistor R1 requires a large amount of rated power since it consumes a large amount of power due to helix voltage Ehel that is applied when transistor Q1 is on.
Furthermore, in the construction shown in FIG. 4, a voltage obtained by dividing helix voltage Ehel using resistors R11 and R12 is applied to anode electrode A when TWT 1 performs a normal operation. Accordingly, as a drawback, the anode voltage is reduced in the normal operation of TWT 1, thereby decreasing the maximum gain of TWT 1.